1. Field of the Invention
This invention relates to a process for the liquefaction of coal wherein the coal is initially contacted with a solvent to form a slurry and the slurry is then hydrogenated, and more particularly to a process for the production of the solvent which can be used to form the initial coal slurry.
2. Description of the Prior Art
The chemical treatment of coal, which is a solid material having a usually high molecular weight, to produce light and heavy oils, which are liquids having low molecular weights, by subjecting the coal to a hydrogenation treatment at elevated temperatures and pressures is well known. Such a process is known as coal liquefaction. Due to the fact that the coal is a solid and thus cannot be easily supplied in a continuous fashion and at a fixed flow volume to the high pressure treatment apparatus, which may be operating at pressures of about 200 kg/cm.sup.2, the coal is usually first pulverized and then contacted with a suitable solvent so as to form a slurry. Such a slurry can more easily be supplied to the high pressure treatment apparatus in a controlled manner.
The utilized solvent must have an acceptable viscosity (such as that of a medium oil) and it must be capable of effectively dispersing the coal and forming a stable slurry (if a stable slurry is not formed and it easily separates into solid and liquid phases, the solid phase (coal powder) will settle in the pipes of the treatment apparatus and interfere with the continuous liquefaction operation). In addition, a suitable solvent will be able to uniformly disperse and stabilize the products emerging from the hydrogenation treatment apparatus. Finally, the solvent should have the capacity to donate hydrogen, thereby not only helping create the initial coal slurry but also help in achieving a complete hydrogenation of the coal in the high pressure treatment apparatus. In this latter regard, if the solvent has an insufficient ability to donate hydrogen for reaction with the coal in the high pressure treatment apparatus, either the amount of coal contained per unit of solvent added to the treatment apparatus must be reduced, or else additional amounts of hydrogen gas must be supplied to the high pressure treatment apparatus. The former alternative is of course quite uneconomical as far as the overall coal liquefaction process is concerned, and the latter is quite dangerous since the apparatus is usually operating at temperatures of 400.degree. to 450.degree. C. and pressures of 100 to 200 kg/cm.sup.2. Pursuing this latter alternative thus requires the use of very highly advanced processing techniques which would be desirable to avoid, if possible.
One conventionally used solvent for coal liquefaction is the heavy liquid which is generated in the coal liquefaction process, this heavy liquid being recirculated to contact new coal either in unmodified form or after having been modified by a single hydrogenation treatment. This heavy liquid has the proper viscosity and possesses a high affinity for coal and thus when mixed with finely pulverized coal it will create a stable coal slurry. However, the heavy liquid in unmodified form does not possess a sufficient ability to donate hydrogen to make it a totally suitable solvent.
In order to increase the ability of such heavy liquid to donate hydrogen, it can be subjected to a hydrogenation treatment. During this treatment hydrogen donating substances will be generated in the heavy liquid. However, the heavy liquid which results from coal liquefaction is a mixture of many components, some of which will not convert into hydrogen-donating substances and, in any event, the conditions under which the hydrogenation treatment takes place will cause a portion of the heavy liquid to break down into gases (methane and ethane). Thus, the yield of hydrogen-donating substances in the solvent will be at most 30%. Methods so far suggested for the liquefaction of coal, including that taught by JA-OS 1202/53, laid open for public inspection on Jan. 9, 1978 have not yet been proven to be economical, safe or operationally stable.
It is therefore an object of this invention to provide a process for the liquefaction of coal by use of a solvent.
Another object of this invention is to provide a process for the production of a solvent for use in the liquefaction of coal.
Still another object of this invention is to provide a process for the production of a solvent which permits the liquefaction of coal to be obtained in high yields.
A further object of this invention is to provide a process for the production, in high yields, of a solvent for use in the liquefaction of coal.